Such medication delivery devices may have applications if persons without formal medical training, i.e. patients, need to administer an accurate and predefined dose of a medicinal product. In particular, such devices may have application where the medicinal product is administered on a regular or an irregular basis over a short-term or a long-term period.
These circumstances set a number of requirements for medication delivery devices of this kind. The device should be robust in construction, yet easy to use in terms of the manipulation of the parts, understanding by a user of its operation and the delivery of the required dose of medication. Dose setting should be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be cheap to manufacture and easy to dispose of (preferably being suitable for recycling). To meet these requirements the number of parts required to assemble the device and the number of material types the device is made from should be kept to a minimum.
User operated medication delivery devices are known from U.S. Pat. No. 7,316,670, for example.
Additionally, if the medication is a fluid or a fluid comprises the medication, there are often gaseous inclusions in the fluid, like air bubbles, for example. Such inclusions may be dangerous for the user's health, if they are not removed from the fluid before administration. Also, some elements of a medication delivery device, like a needle, for example, may be filled with gas before the very first dose of medication is dispensed from the device. This may also be dangerous for the user's health, if the gas is not removed. In order to minimize the risk of health damages, gas can be removed from the fluid before the first administration of medication (“priming of the device”). This can be done, for example, by expelling a small dose of the fluid (“the priming dose”) from the device while pointing the device in a predetermined orientation, for example needle-up. Thus, gas may be removed from fluid and needle and the risk of injecting a gas into the user is minimized and/or manufacturing tolerances may be removed and/or an initial air gap between the piston rod and the piston of the cartridge may be removed. Of course, the amount of expelled fluid cannot be used for subsequent administration of the medication. As medication is usually rather expensive, the priming dose should be kept at a minimum.
Furthermore, the size of the device, for example its length, should be kept as small as possible, because medication delivery devices, in particular the ones for regular administration of medication, are often carried along by the user and therefore the space required for storing the device matters to the user. In addition, smaller devices are more appealing than larger ones.